The natural gas deposits being exploited contain increasing quantities of nitrogen. This is notably because fields that are rich enough for no enrichment treatment to be needed before the gas is commercialized are becoming exhausted and increasingly rare.
These sources of natural gas often also contain helium. The latter can be put to commercial use by performing a pre-concentration before final treatment and liquefaction.
Unconventional resources such as shale gas deposits also share the same problem set: in order to make them commercially viable, it may prove necessary to increase their calorific value by means of a pretreatment that involves removing nitrogen from the raw gas.
U.S. Pat. No. 4,778,498 describes a double column used for a denitrogenization of natural gas.
M. Streich's “Nitrogen Removal from Natural Gas” presented at the ICR12 in Madrid in 1967 explains how to use a turbine to expand the natural gas that is to be separated in a double denitrogenization column.
Units for the removal of nitrogen from natural gas generally treat gases which come directly from wells at a high pressure. After the removal of nitrogen, the treated gas needs to be returned to the network, often at a pressure close to the pressure at which it entered it.
The removal of nitrogen from natural gas in most instances calls for cryogenic distillation techniques which take place at pressures lower than the pressures of the sources. For example, the sources may be at pressures of the order of 60 to 80 bara, whereas cryogenic separation is performed at pressures varying from 30 bara to a pressure slightly higher than atmospheric pressure. In general, the natural gas purified of its nitrogen is produced at low pressure and it needs to be pumped and/or compressed in order to introduce it into the network.
In order to adapt the thermal and energy balance sheets and minimize the operating costs of the unit, the natural gas purified of nitrogen may be produced at different pressure levels as it leaves the cold box. The various streams are then compressed by external compression until they reach the desired pressure.
In addition, distillation at pressures in excess of 12 bara is generally ill suited to the use of structured packing because of “washer” phenomena associated with the fact that the densities of the gases and liquids passing through the columns are very closely spaced, entailing the use of trays for these pressure levels.